xfs_growfs: allow mounted device node as argument

xfs_growfs: allow mounted device node as argument

Up until:

 b97815a0 xfs_growfs: ensure target path is an active xfs mountpoint

xfs_growfs actually accepted a mounted block device name as the
primary argument, because it could be found in the mount table.

It turns out that Ansible was making use of this undocumented behavior,
and it's trivial to allow it, so put it back in place and document
it this time.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>